1. Field of the Invention
The present invention relates to a television broadcast receiving apparatus, and particularly to reduction of SN (Signal to Noise) ratio deterioration due to an interfering signal interfering with a channel being watched in the case of receiving digital terrestrial television broadcasting.
2. Description of the Background Art
Regarding the terrestrial television broadcasting in Japan, analog broadcasting is officially scheduled to be terminated and completely shifted to digital broadcasting in July 2011. Currently, simulcast (simultaneous broadcasting) of simultaneously providing analog broadcasting and digital broadcasting is performed as a transitional measure. Therefore, for receiving apparatuses receiving broadcasts in the transitional period in which the broadcasting system is to be changed, there is an increasing demand for resistance to interferences such as interference of an analog broadcast channel with a digital broadcast channel being received, contrary interference of a digital broadcast channel with an analog broadcast channel being received, and interference of another digital broadcast channel with the digital broadcast channel being received. Even when the analog broadcasting has been terminated and only the digital broadcasting is provided, the broadcast waves for the digital broadcasting are permitted to be arranged with a channel spacing of 6 MHz without vacant frequency band, and therefore, the demand for resistance to interferences between channels will not be mitigated in the future as well, due to increase of the broadcast stations.
FIG. 19 is a block diagram showing a conventional digital broadcast receiving apparatus 90. In the following, a general operation of the conventional digital broadcast receiving apparatus will be described.
A broadcast signal from a broadcast station is input to an input terminal 1, passed through a bandpass filter 2 allowing a frequency of a desired channel to pass, and amplified by a variable gain amplifier 3 to an appropriate value. After this, the signal is passed again through a bandpass filter 4 tuning the frequency of the desired channel, and input to a mixer 5. Mixer 5 constitutes a frequency conversion unit 21 together with a local oscillator 20. The input signal to mixer 5 is mixed with an output signal of local oscillator 20, and is output as a sum and difference signal generated by mixing in the frequency range. An intermediate frequency filter 6 allows only a difference component of the output signal of mixer 5 to pass. The resultant output signal is called intermediate frequency signal. The intermediate frequency signal is first amplified by an amplifier 7, and then input to a SAW (Surface Acoustic Wave) filter 8 and also detected by an AGC (Auto Gain Control) detector 18. The resultant detection output signal controls the degree of amplification of variable gain amplifier 3. Here, control of the degree of amplification of variable gain amplifier 3 will be described in more detail. When the reception input level to input terminal 1 increases and the output level of amplifier 7 exceeds a specified value, the gain of variable gain amplifier 3 is reduced. When the output level of amplifier 7 goes below the specified value, the gain of variable gain amplifier 3 is increased. In this way, a gain control operation of keeping constant the output level of amplifier 7 is performed. Accordingly, the resistance to interference when the reception input level is high and a favorable SN ratio when the reception input level is low are achieved together.
Next, the intermediate frequency signal with the frequency band limited by SAW filter 8 is appropriately amplified by a variable gain amplifier 9, passed through a bandpass filter 10, and converted by an A/D (Analog to Digital) converter 11 to an input signal to a demodulation unit 12. A level detector 16 detects the output signal of A/D converter 11. From the resultant detection output signal, a control signal is generated by an AGC control signal generation unit 17, and by the control signal, variable gain amplifier 9 adjusts its degree of amplification so that the input level to demodulation unit 12 is made constant. Demodulation unit 12 includes an SN ratio detection unit 13. The detection value of SN ratio detection unit 13 is transmitted to an operation unit 14 and may be used for an on-screen display capability of a receiver (not shown). A storage unit 15 may be used for temporary storage for the above-described display capability or the like. Operation unit 14 transmits through a bus 22 oscillation frequency setting data to a PLL (Phase Locked Loop) unit 19 for controlling the oscillation frequency of local oscillator 20.
Here, the output level of amplifier 7 when the above-described gain control operation for variable gain amplifier 3 is started is generally called takeover point. FIG. 20 is provided for explaining the takeover point, and illustrates that variable gain amplifier 9 reaches a specified gain attenuation amount and thereafter variable gain amplifier 3 starts a gain attenuation operation. This takeover point is set in a trade-off manner to an appropriate value in consideration of the interference characteristic and the SN ratio, and the set value does not change in most cases. The actual reception environment, however, varies depending on the reception area and the condition under which the receiver is installed, for example, and the set value is not necessarily optimum. In such a case, the level of the interference characteristic deteriorates to be lower than a level expected in advance, and the deterioration of the interference characteristic could cause the SN ratio to deteriorate as well.
In order to lessen these disadvantages, Patent Document 1 (Japanese Patent Laying-Open No. 2001-102947 published on Apr. 13, 2001) and Patent Document 2 (Japanese Patent Laying-Open No. 2006-50585 published on Feb. 16, 2006) propose respective techniques. The technique of Patent Document 1 changes, depending on the reception quality, a switch level value between a gain control operation for an intermediate frequency amplification circuit and a gain control operation for a high-frequency amplification circuit, and sets the takeover point to an optimum point all the time. The technique of Patent Document 2 detects the reception state for each channel, sets the takeover point to the one that provides an optimum reception state for a detected input high-frequency signal for each channel, and updates and sets the takeover point to the one that provides an optimum reception state at the timing that does not influence watching.
As seen from the above, regarding ensured reception performance by means of gain control, the conventional techniques are generally classified into the one that fixes the takeover point to a tolerable limit for actual use, and the one that provides optimum setting in accordance with the reception channel and the reception quality. Meanwhile, the reception performance is considerably influenced by, in addition to the reception environment condition as described above, variation of the characteristics of the receiving apparatus itself. In particular, bandpass filters 2, 4 included in receiving apparatus 90 of FIG. 19 are tuned for each reception channel for generating the intermediate frequency signal between them and local oscillator 20. Since this tuning varies depending on the reception channel, the frequency response of a reception channel could be smaller than the frequency response of an adjacent channel in an extreme case. In the case of Japan for example, when a broadcast signal with a center frequency of 509.142857 MHz is input and an intermediate frequency signal of 57 MHz is to be obtained, the frequency is converted with the oscillation frequency of local oscillator 20 that is the sum of the aforementioned frequencies of 566.142857 MHz. Although the tuning frequency of bandpass filters 2, 4 should be tuned to 509.142857 MHz that is the center frequency of the broadcast signal, when the tuning frequency is for example 515.142857 MHz, the peak of the intermediate frequency signal output characteristic is deviated to 54 MHz (hereinafter tracking error), and the frequency response of the adjacent channel is enhanced. In such a case, with the sole takeover point setting of the conventional techniques, not only optimization of the reception characteristics but also an ensured tolerable limit for actual use could be difficult to achieve.
FIG. 21 illustrates an example of the above-described case, in which an interfering signal of an analog broadcast is present adjacent to a desired reception signal of a digital broadcast. In FIG. 21, the horizontal axis of each graph represents the frequency and, for input and output signals both, the center frequency is indicated as a reference (0 Hz). The vertical axis of each graph represents the amplitude of a signal that is indicated as a relative level with respect to a reference (0 dB) of an input signal level.
In FIG. 21(a), a difference in amplitude level between the desired signal of the digital broadcast and the interfering signal of the analog broadcast is 15 dB, and the signals are input to input terminal 1 of television broadcast receiving apparatus 90 in FIG. 19. In FIG. 21(b), with respect to the frequency of the desired reception channel, there is no tracking error between the tuning frequency of the bandpass filters 2, 4 and the oscillation frequency of local oscillator 20. Accordingly, as shown in FIG. 21(c), the output signal spectrum of bandpass filters 6, 10 does not deviate from the center frequency of the intermediate frequency signal and, with respect to the amplitude level of the center frequency, the desired signal is not attenuated and the interfering signal is suppressed. In contrast, in the case as shown in FIG. 21(d) in which there is a tracking error between the tuning frequency of bandpass filters 2, 4 and the oscillation frequency of local oscillator 20 when the desired reception channel is received, the output frequency characteristic of bandpass filters 6, 10 deviates from the center frequency of the intermediate frequency signal. Then, regarding the output signal spectrum of bandpass filters 6, 10, the desired signal as shown in FIG. 21(e) is attenuated with respect to the amplitude level of the center frequency and the interfering signal is not suppressed. Thus, the difference of the amplitude level increases, which leads to deterioration of the SN ratio.